Operational amplifiers, or, “op-amps,” are well-known circuits used in a variety of applications. For example, operational amplifiers are used as active filters, oscillators, voltage and current amplifiers, integrators and differentiators, analog-to-digital converters (“ADCs”) and digital-to-analog converters (“DAC's”), to name a few. Desirable characteristics of op-amps include high open loop gain, high input impedance, low output impedance low offset and low offset drift.
However, one problem op-amps suffer is called “offset error.” This effect occurs because of the inherent lack of precision in the matching of the op-amp's components, including the two differential input transistors. Ideally, the op-amp has a zero output voltage for zero input voltage. But, when the op-amp's input transistors are unmatched, the op-amp may have a non-zero output voltage for zero input, which is the offset error. The voltage applied to the differential input that makes the output voltage zero is called the “input offset voltage.” This offset error can have an adverse effect in any circuit in which the op-amp is used, if compensation is not provided for it.
In precision applications, it is necessary for the offset error to be minimized, and numerous approaches to that problem have been proposed and implemented. However, even after compensating for the offset error, the factors giving rise to it can vary with varying temperature, giving rise to a variation in the offset error with temperature, called “offset drift.” This offset drift can make compensation for offset error that is static with respect to temperature inadequate in precision applications.
Approaches to compensate for offset drift have therefore been proposed. One approach is disclosed in U.S. Pat. No. 6,396,339, which issued to Karl H. Jacobs on May 28, 2002, and was assigned to Texas Instruments Incorporated. In the technique disclosed therein, input offset voltage is compensated by balancing the operational amplifier over the operating temperature range after the device has been initially trimmed. Their operational amplifier employs a lower input offset voltage, which remains low over the operating temperature range without a separate temperature compensation circuit. They provide a separate trim device for each current path of the circuit to maintain symmetry. Thus, the current paths of the differential circuit have the same leakage current upon temperature excursions. Ideally, the leakage current will occur in both current paths of the differential circuit and maintain circuit balance.
Another example is disclosed in U.S. Pat. No. 4,490,713, which issued to Andrij Mrozowski et al. on Dec. 25, 1984, and was assigned to Burr-Brown Inc. In the technique disclosed therein, a solution to offset drift is described in the context of an ADC having offset drift, a portion of which may be contributed by an operational amplifier therein. They employ a differential temperature sensor that generates a temperature-dependent voltage, Vt. During calibration at ambient temperature, that voltage is applied to the ADC input to obtain a sixteen-bit digital representation of Vt, which is stored. Then, in use, after an analog sample is converted the differential temperature sensor is applied to the input again, to obtain another sixteen-bit digital representation of Vt for whatever the present temperature is. The difference between the two values is used to do a look-up in a gain and offset drift storage register, which is preprogrammed to contain the amount of gain and offset drift that occurs as a function of temperature change. The sixteen-bit digital representation of the analog sample is compensated by that amount to obtain the final digital value for the analog sample.
It is therefore desirable to have an op-amp including compensation for offset drift that is effective over an intended temperature range, while at the same time offering a minimal performance penalty for the op-amp.